1. Field of the Invention
The present invention relates to a moving image noise reduction processing apparatus, a computer-readable recording medium on which a moving image noise reduction processing program is recorded, and a moving image noise reduction processing method for reducing noise in a digitalized time-series image signal.
2. Description of the Related Art
An image pickup system includes components such as an image sensor and an analog circuit and an analog-to-digital converter associated with the image sensor. A video signal provided from the image pickup system typically contains noise components. The noise components can be broadly divided into two categories: fixed pattern noise and random noise.
The fixed pattern noise is noise, such as defective pixels, caused primarily by the image sensor.
The random noise, on the other hand, is generated in the image sensor and the analog circuit and has a characteristic similar to a white noise characteristic.
For the random noise, Japanese Patent Application Laid-Open Publication No. 2001-157057 discloses a technique in which statically given constant terms a, b and c and a signal level D converted to a concentration value are used to express an amount of noise N in function form, N=abcD, an amount of noise N related to the signal level D is estimated from the function, and a filtering frequency characteristic is controlled on the basis of the estimated amount of noise N. With this technique, noise reduction processing adaptive to the signal level is performed.
Japanese Patent Application Laid-Open Publication No. 2005-303802 describes a technique in which a noise model is simplified and the simplified model is used to calculate an amount of noise.
On the other hand, there is a processing technique for reducing random noise in moving images that uses correlation in a time axis direction to perform cyclic noise reduction processing.
For example, Japanese Patent Application Laid-Open Publication No. 2000-209507 describes such a technique in which a difference between an image signal of a current frame (or a current field; the following description will be provided with respect to only frames as representative of frames and fields) and an image signal of a previous frame is processed and, on the basis of a signal value resulting from the processing, a feedback factor in cyclic noise reduction processing is controlled. In broad terms, the feedback factor represents a rate of contribution of a previous frame to noise reduction processing of the image signal of the current frame. More specifically, if the signal value resulting from the difference processing is small, the signal is determined to be random noise, and the feedback factor is increased (the rate of contribution of the previous frame is increased) to obtain an image signal containing a reduced amount of noise. On the other hand, if the signal value resulting from the difference processing is large, the signal is determined to be a motion component and the feedback factor is reduced (the rate of contribution of the previous frame is reduced) to obtain an image signal containing a reduced amount of residual image. The technique has advantages that the amount of computation is small and costs can be kept low because control is performed based on a signal value resulting from the difference processing.